Right…
Note I'm going off the background fiction et al here, not the wargame, so there may be some disagreement.
Hrud Fusil: I'd change the weapon class to "exotic", but otherwise fine.
Kroot Long Rifle: Do you mean the long rifle, which is a sniper weapon which doesn't have the blades, or the normal rifle? If you mean the normal rifle, I'd rename it the Kroot Rifle, and add a note that the Kroot Rifle counts as an axe in close combat, not a spear, as if you look at the design it's for swinging, not stabbing.
Gauss Flayer: Bump up the range to 90m and the damage to 1d10+6 R (as it behaves closer to a Rending weapon, with the high chance of limb removal). Moreover, it isn't a weapon which needs to be reloaded in the hands of a Necron,as it draws it power from the same technobabble source they do, and no other race knows enough about it to reload it.
Xenach Death Arc: Give it the Shocking special rule, but otherwise fine.
Eldar Shuriken Catapult: Oh dear. Here is where we really diverge. The Shuriken Catapult is a terrifying weapon in the fiction, with a horrible rate of fire, massive clip and the ability to shred things very well. In 2nd edition 40k, which is more representative of the background, it was slightly better than a boltgun. Thus, here is my suggested stat line.
Eldar Shuriken Catapult
Type: Basic (Shuriken)
Range: 90m
RoF: -/10/30
Damage: 1d10+4 Rending (Itta cutta da limbsa!"
Pen: 1
Clip: 200
Rld: Full
Special Rules: Tearing
Weight: 3kg
Tau Pulse Rifle: Another divergence. In background, a pulse rifle has a similar effect to a high-powered lasgun shot, so I suggest we have it as the base. Yes, it may be described as plasma, but fire is low energy plasma. It's just an adjective. It's got a better range than a lasgun, but is worse at close range. Moreover, the Pen value is being used too freely; they can be stopped quite well by flak armour. In fact, it is close to an long las. Thus:
Tau Pulse Rifle
Type: Basic (Pulse)
Range: 120m
RoF: S/2/-
Damage: 1d10+4 Energy
Pen: 1
Clip: 20
Rld: Full
Special Rules: Accurate, Unmanoeuvrable *
Weight: 6kg
*Bonus for point blank (+30) reduced to +10 BS
Tau Pulse Carbine: Similarly, most of my previous objections apply.
Tau Pulse Carbine
Type: Basic (Pulse)
Range: 65m
RoF: S/4/-
Damage: 1d10+4 Energy
Pen: 1
Clip: 20
Rld: Full
Special Rules: Underslung Grenade Launcher (only usable with Photon Flash Grenades)
Weight: 4kg